1. Field of the Invention
This invention relates generally to a statistical process control gaging apparatus and, more particularly, to a sensor for accurately determining the location and position of a discontinuity, such as a hole, in a workpiece.
2. Description of the Related Art
In many manufacturing situations involving the mass production of precision components whether they be formed of sheet or cast metal, wood, plastic or any other suitable material of construction, it is essential to accurately locate and position numerous holes in the component or workpiece. The holes may allow the workpiece to be mounted to a support surface or may enable an accessory component to be secured to the workpiece. If the workpiece is to function properly in a larger assembly or if the accessory component is to be securely mounted to the workpiece, it may be necessary to have several holes precisely located and positioned relative to the workpiece. The location and position of the holes may be determined with respect to an edge of the workpiece, a critical point on the workpiece, or another hole formed in the workpiece.
Historically, workpiece quality control procedures required time-consuming, laborious, manual calculations and measurements which demanded considerable skill and precision to perform. As a result, special dedicated gaging devices were developed. Such gages employ a fixture uniquely patterned after the workpiece. The fixture is formed with holes that are aligned with the holes formed in the workpiece. A pin is inserted into the hole formed in the fixture. If the workpiece hole has been formed at the desired location, the pin is simultaneously and readily received within the workpiece hole. If the hole has been formed at a location which falls outside the range of allowed tolerances, the workpiece hole is not sufficiently aligned with the fixture hole for the pin to be received within the workpiece hole. Thus, the gaging devices that are presently known merely function to indicate whether the workpiece hole has been formed at a location which is within the range of allowed tolerances. Hence, such devices are sometimes referred to as "go pins" because the workpiece hole is a "go" if the pin is received within the workpiece hole, indicating that the hole has been placed within allowed tolerances. On the other hand, the hole is a "no go" if the pin is not received within the workpiece hole, which indicates that the actual hole location is outside the range of allowed tolerances.
The presently known hole location devices lack the ability to provide variable data; that is, data describing not only the dimensional extent to which the hole has been displaced relative to the desired location but also the direction in which the displacement has occurred. Variable data is valuable in improving workpiece quality control procedures because the data may be stored, analyzed and displayed with a computerized data management system connected to the gage. The data may be used to detect the tendency for the hole location to drift or shift during a production run, for example, if the hole forming equipment becomes out of align, thereby indicating the need for equipment adjustment. A feedback loop connecting the hole location device with the hole forming equipment may provide for automatic correction of the forming equipment.
Variable data is also extremely useful in determining the location of a hole having non-uniform tolerances; that is, tolerances which vary with respect to an axis. Coordinate measurement machines are presently available for assisting in determining the location of the hole with respect to two axes. However, such machines are not only cumbersome and difficult to use, relative to "go pins", but are prohibitively expensive. Presently known gaging systems are not only elaborate, complex and costly but require a separate gage for each workpiece hole. For some manufacturers, the cost of providing multiple gages is prohibitive.
Thus, there is a need for a hole location apparatus that is adapted to provide variable data along two axes. As explained above, variable data enhances workpiece quality control procedures because separate dimensional and directional information can be collected along two axes for each hole. This not only refines the extent to which the hole location can be measured but can provide information regarding the tendency for the hole location to drift during a production run. It would also be advantageous if such devices were adapted to sequentially measure the position and location of multiple holes. Because workpieces often contain numerous holes, the position and location of which must be individually verified, it would be advantageous if a single fixture having multiple bores but adaptable for use with only a single gage could be constructed. In such a system, the gage would be receivable within each of the fixture bores and registerable with each of the workpiece holes to determine their location and position. In this arrangement, the workpiece would be mounted to the fixture and a single gage would be inserted within each bore sequentially to calculate the position and location of the associated workpiece hole. Thus, only a single gage rather than several gages would be required for collecting variable data along two axes for each workpiece, thereby resulting in a significant cost reduction for carrying out the quality control procedure.